In world where only once carrier could carry a 950, the entire Lumia 950 & 950 XL situation would actually be a lot more beautiful if Verizon was that carrier and Microsoft offered a monthly installment plan for the unlocked 950 & 950 XL to make up for people who want to use it on ATT & can't buy full price all at once (users who would go with the NEXT plan).
This way the phone is available to all gsm carriers plus the largest U.S carrier often referred to as "Big Red" without AT&T users being forced to pay one way. Although this all would require interested 950 buyers not too tech knowledgeable to wise up on unlocked phones, installment plans, sim cards, etc. rather than just going to the ATT store and looking for what's the latest & greatest.
But these devices are "for the fans" anyway after all, aren't they? The complicated way Lumia carrier set ups have been over the years makes me want to assume that the Lumia users that remain in the U.S are probably mostly "the fans" anyway unless they are folks inly on Windows for the low priced phones in which case they probably wouldn't buy the 950 anyway. So my proposed scenario really wouldn't push any users away other than people who weren't going to buy these phones anyway.
Not saying this was at all a possible option for MS as Verizon probably has never wanted anything to do with these phones in the first place. So Just a thought...